1. Field of the Invention
The present invention relates to a wavelength conversion laser device, more particularly, which has a non-linear crystal for generating a second harmonic wave and is configured to rotate the non-linear crystal for phase-matching with a fundamental wave.
2. Description of the Related Art
Recently, there has been a rising demand for a semiconductor laser in various displays and optical recording devices. Especially, the semiconductor laser has found broader applications in the displays to implement full color.
This has considerably called for a laser consuming lower power but having high output in a visible ray region.
In order to obtain red light, an AlGaInP- or AlGaAs-based semiconductor laser has been employed with relative ease. However, to produce green light or blue light, a group III semiconductor is very difficult to grow compared to other semiconductor materials due to unique lattice constant and thermal coefficient thereof. The group III semiconductor is very high in crystal defects such as dislocation, thereby undermining reliability of the laser and shortening lifetime thereof.
To overcome this problem, methods for converting a wavelength through non-linear characteristics have been proposed. In one of the methods, a diode-pumped solid-state (DPSS) laser has been adopted, drawing attention. For example, light from a pump laser diode with a wavelength of 808 nm is made to enter a crystal such as Nd:YAG to obtain a wavelength of around 1,060 nm. Then a frequency of the light is doubled using the non-linear crystal, thereby producing green light with a wavelength around 530 nm.
In the case of the DPSS laser device, the non-linear optical crystal such as a second harmonic generation crystal exhibits a temperature-induced change in refractivity according to a crystal orientation. Accordingly, temperature alters an incidence angle for phase-matching, i.e., optimum wavelength conversion efficiency. This has called for a method for maintaining the wavelength conversion efficiency of the non-linear optical crystal within a temperature range.
Conventional methods involve a thermo-electric cooler (TEC) using a Peltier device and a heat radiating structure. This, however, increases power consumption or the size of the system. To overcome this drawback, U.S. Pat. No. 6,614,584 by Sergei et al. discloses a method for displacing the non-linear optical crystal to have an incidence angle meeting optimum phase-match conditions by monitoring the output of light and feeding it back.
FIG. 1 is a schematic view illustrating a wavelength conversion laser device proposed in the document.
The wavelength conversion laser device 10 shown in FIG. 1 includes a laser optical source 1 and a non-linear optical crystal 5 for converting wavelength light of the laser light source 1 into desired wavelength light.
In the wavelength conversion laser device 10, a portion of an output of the wavelength light propagates to a second beam distributor 2b by a first beam distributor 2a. The second beam distributor 2b splits the light into horizontal and vertical components. Here, the split wavelength components can be transferred to the first and second location detectors 7a and 7b through a spectral filter 4. The first and second location detectors 7a and 7b detect phase mismatch of the light transferred and in turn, a controller 8 causes a displacement  to the non-linear optical crystal 5 through a rotator in order to have an incidence angle for obtaining optimal output conditions.
As described above, the wavelength conversion laser device 10 shown in FIG. 1 monitors the output of light converted by the non-linear optical crystal 5, and feeds back a phase mismatch degree resulting from the present conditions, e.g., temperature. Then the wavelength conversion laser device 10 mechanically causes a displacement to the optical crystal 5, thereby maintaining maximum optical conversion efficiency.
However, in the wavelength conversion laser device 10, a final output position is changed according to the displacement γ of the non-linear optical crystal 5. More specifically, as shown in FIG. 1, when the non-linear optical crystal 5 is displaced (indicated with a dotted line), the exiting position of light is displaced by Δα from OUT1 to OUT2.
The displacement in the exiting position degrades precision of the device, rendering it hard to configure an optical system using output light. Especially, this proves very serious for a miniaturized product such as a mobile projector which has gained attention as an application for the laser device. Furthermore, an exiting position adjusting structure needs to be simply configured without adopting a complicated optical system to meet requirements for the miniaturized product.